Conventional Cellular Systems. The present day cellular mobile telephone system developed due to a large demand for mobile services that could not be satisfied by earlier systems. The cellular system “reuses” frequency within a system of cells to provide wireless two-way radio frequency (RF) communication to large numbers of users. Each cell covers a small geographic area and collectively an array of adjacent cells covers a larger geographic region. Each cell has a dedicated fraction of the total amount of RF spectrum which is used to support cellular users located in the cell. Cells are of different sizes (for example, macro-cell or micro-cell) and are generally fixed in capacity. The actual shapes and sizes of cells are complex functions of the terrain, the man-made environment, the quality of communication and the user capacity required. Cells are connected to each other via land lines or microwave links and to the public-switched telephone network (PSTN) through telephone switches that are adapted for mobile communication. The switches provide for the hand-off of users from cell to cell and thus from frequency to frequency as mobile users move between cells.
In conventional cellular systems, each cell has abase station with RF transmitters and RF receivers co-sited for transmitting and receiving communications to and from cellular users in the cell. The base station employs forward RF frequency bands (carriers) to transmit forward channel communications to users and employs reverse RF carriers to receive reverse channel communications from users in the cell. Conventional forward channel communications are static in that they employ fixed power, at fixed frequencies and have fixed sectors if sectorized antennas are used.
The forward and reverse channel communications use separate frequency bands so that simultaneous transmissions in both directions are possible. This operation is referred to as frequency domain duplex (FDD) signaling. Although time domain duplex (TDD) signaling, in which the forward and reverse channels take turns using the same frequency band is possible, such operation is not part of any widespread current cellular implementation.
The base station in addition to providing RF connectivity to users also provides connectivity to a Mobile Telephone Switching Office (MTSO). In a typical cellular system, one or more MTSO's will be used over the coverage region. Each MTSO can service a number of base stations and associated cells in the cellular system and supports switching operations for routing calls between other systems (such as the PSTN) and the cellular system or for routing calls within the cellular system.
Base stations are typically controlled from the MTSO by means of a Base Station Controller (BSC). The BSC assigns RF carriers to support calls, coordinates the handoff (handover) of mobile stations (users) between base stations, and monitors and reports on the status of base stations. The number of base stations controlled by a single MTSO depends upon the traffic at each base station, the cost of interconnection between the MTSO and the base stations, the topology of the service area and other similar factors.
A handoff between base stations occurs, for example, when a mobile user travels from a first cell to an adjacent second cell. Handoffs also occur to relieve the load on a base station that has exhausted its traffic-carrying capacity or where poor quality communication is occurring. The handoff is a communication transfer for a particular user from the base station for the first cell to the base station for the second cell. During the handoff in conventional cellular systems, there is a transfer period of time during which the forward and reverse communications to the mobile user are severed with the base station for the first cell and are not yet established with the second cell. A typical conventional cellular system has the transfer period designed to be less than 100 milliseconds. Conventional cellular implementations employ one of several techniques to reuse RF bandwidth from cell to cell over the cellular domain. The power received from a radio signal diminishes as the distance between transmitter and receiver increases. All of the conventional frequency reuse techniques rely upon power fading to implement reuse plans. In a frequency division multiple access (FDMA) system, a communications channel consists of an assigned particular frequency and bandwidth (carrier) for continuous transmission. If a carrier is in use in a given cell, it can only be reused in cells sufficiently separated from the given cell so that the reuse site signals do not significantly interfere on the carrier in the given cell. The determination of how far away reuse sites must be and of what constitutes significant interference are implementation-specific details. The cellular Advanced Mobile Phone System (AMPS) currently in use in the United States employs FDMA communications between base stations and mobile cellular telephones.
In time division multiple access (TDMA) systems, multiple channels are defined using the same carrier. The separate channels each transmit discontinuously in bursts which are timed so as not to interfere with the other channels on that carrier. Typically, TDMA implementations also employ FDMA techniques. Carriers are reused from cell to cell in an FDMA scheme, and on each carrier, several channels are defined using TDMA methods.
In code division multiple access (CDMA) systems, multiple channels are defined using the same carrier and with simultaneous broadcasting. The transmissions employ coding schemes such that to a given channel on a given carrier, the power from all other channels on that carrier appears to be noise evenly distributed across the entire carrier bandwidth. One carrier may support many channels and carriers may be reused in every cell.
In space division multiple access (SDMA) systems, one carrier is reused several times over a cellular domain by use of adaptive or spot beam-forming antennas for either terrestrial or space-based transmitters.
TDMA Conventional Cellular Architectures. In TDMA systems, time is divided into time slots of a specified duration. Time slots are grouped into frames, and the homologous time slots in each frame are assigned to the same channel. It is common practice to refer to the set of homologous time slots over all frames as a time slot. Each logical channel is assigned a time slot or slots on a common carrier band. The radio transmissions carrying the communications over each logical channel are thus discontinuous. The radio transmitter is off during the time slots not allocated to it.
Each separate radio transmission, which should occupy a single time slot, is called a burst. Each TDMA implementation defines one or more burst structures. Typically, there are at least two burst structures, namely, a first one for the initial access and synchronization of a user to the system, and a second one for routine communications once a user has been synchronized. Strict timing must be maintained in TDMA, systems to prevent the bursts comprising one logical channel from interfering with the bursts comprising other logical channels in the adjacent time slots. When bursts do not interfere, they are said to be isolated. Burst-to-burst isolation may be quantified in several ways. One measure is the minimum signal-to-interference ratio between the burst intended for a time slot and the bursts intended for the preceding and following time slots, said minimum ratio being taken over the information-carrying length of the burst in question. If this ratio never drops below an implementation-specific value, the burst is said to be isolated from the adjacent bursts. In the event that this safety margin is violated, another measure of isolation is the fraction of the total burst for which the margin is violated. This measure may be a weighted measure if the importance of data or the degree of coding protection afforded the data varies over the length of the burst. Data variation over the burst is typical in TDMA implementations.
The isolation of one burst from the preceding and following bursts is crucial for TDMA systems. The defined burst structures are constructed to assist in the isolation process. A burst theoretically cannot completely fill its allotted time slot because radio transmitters neither commence nor cease transmitting instantaneously. TDMA implementations therefore allow time for radio signal strength to ramp up and to ramp down in each of the defined burst structures. During normal communications to and from a synchronized user, each burst does not quite fill its specified time slot. A guard period, TG, is inserted before and/or after each normal burst to allow for timing mismatches, multipath delays, and inaccuracies within the system. The initial synchronization bursts for accessing the system fill even less of a time slot than do normal bursts. The long guard period, TLG, for synchronization bursts is used to overcome the timing mismatches caused by the unknown separation between a user and the base station.
Within a cell, the base station maintains a time base which users synchronize to during initial access. User synchronization to a particular base station is achieved using synchronization bursts sent periodically on a specific carrier by that base station and the reply synchronization bursts sent by the user. Those reply transmissions will arrive delayed at the given base station by the propagation time for radio signals over the separation between the user and the given base station. The separation is generally unknown because the users are mobile. Not only is a burst delayed, but in the cellular multipath environment, multiple copies of the burst are received over some delay spread corresponding to multipath reception over reflected paths of varying lengths. A digital signal processing technique known as equalization is commonly used in RF communications to correct for multipath delay spreading and fading. After equalization, the base station can measure a single skewing delay time for the user synchronization burst. The base station then commands the user to correct for this delay time by time advancing the user bursts by an equal time interval. Thus each individual user has a time base set by the base station to ensure that the transmissions from all users will arrive back at the base station in synchronization with the base station time base.
These burst structures are detailed for two typical conventional cellular TDMA implementations. Under the European-defined “Global system for mobile communications” (GSM) standard, which is substantially copied in the United States within the PCS 1900 standard, each RF carrier occupies 200 kHz of bandwidth. Each carrier is divided into time slots of 577 μs, organized into 8-slot frames lasting 4.615 ms. Each physical channel receives one time slot per frame, and a variety of logical channels may be constructed on a physical channel. The digital coding scheme used in GSM has a bit length of 3.69 μs. A normal speech burst consists of 148 bits of information followed by 8.25 bit periods of guard time. Thus for GSM, the standard is TG=8.25 bit periods=30.44 μs. The reverse channel synchronization (in GSM terminology, the random access) burst has 88 bits of signaling information followed by 68.25 bit periods of guard time. Thus for GSM, the TLG=68.25 bit periods=252 μs.
Under the IS136 TDMA standard, each RF carrier occupies 30 kHz of bandwidth. Each carrier is divided into time slots of 6.67 ms, organized into 6-slot frames lasting 40 ms. Each logical channel receives two time slots per frame. The bit length for IS136 is 20.58 μs. A normal reverse channel burst consists of 6 guard bit periods, 6 ramp bit periods, and 312 bits of mixed control signaling and data. Thus for IS136, TG=6 bit periods=123.48 μs. The reverse channel synchronization burst has a longer guard period of 38 bit periods, so that TLG=38 bit periods=782.0 μs for IS136
The TG and TLG are principally used to counteract the effects of propagation path travel time and delay spread. These effects are collectively referred to as user time skew. Given the speed of light as approximately 3×108 m/s, the maximum path lengths, DG and DLG, are derived for which the guard periods will compensate for the user time skew. For GSM, TG=30.44 μs so that DG=9.13 km. Similarly, TLG=252 μs so that DLG=75.6 km. For IS136, TG=123.48 μs, DG=37.5 km, TLG=782.0 μs and DLG=234.6 km. As an additional constraint for GSM, the maximum timing advance which can be commanded for a user is 64 bit periods=236.2 μs, which equates to 70.85 km. Both the GSM and IS136 TDMA cellular implementations use equalization and convolutional coding to correct for multipath delay spreading of a burst. However, if delayed versions of a burst arrive more than TG late, they may interfere with the burst from another source intended to arrive in the following slot. Typically, signals arriving on paths many microseconds longer than the shortest path, which is the straight-line path, are received with much lower strength than the earlier signals, and the burst-to-burst interference is thus tolerable in some circumstances.
In general for all current TDMA implementations, a maximum cell radius exists beyond which it is not possible to synchronize users. The maximum synchronization radius, Rsynch-max is found by dividing DLG by 2, since the delay found for the initial synchronization burst is equal to the round-trip travel time from the base station to the user and back. For longer travel time, the initial synchronization bursts are not completed prior to the end of the time slot in which they are to be received, and the system will not recognize the communication as a request for synchronization. Thus Rsynch-max=35.4 km for GSM implementations and Rsynch-max=117.3 km for IS136 implementations. These distances define the cell sizes. If larger cells are desired, then channel assignment schemes which leave empty time slots between all pairs of time slots in use can be employed, but such operation is at the expense of capacity.
In a mobile wireless network, mobile stations (MS) are typically in communications with one base transceiver station (BTS) through up and down radio links. Such ground-based radio links suffer from strong local variations in path loss mainly due to obstructions and line-of-sight attenuation. As MS move from one point to another, their signal path losses go through shadow fading fluctuations that are determined, among other things, by the physical dimension of the obstructions, antenna heights and MS velocity. These variations in path loss, must be taken into account in the design of the uplink and downlink radio link resource allocation.
While communicating with a specific host BTS, MSs are frequently within the communications range of other BTSs. Statistically, due to the distribution of physical obstructions, the shadow fading path loss fluctuations to such other BTS tend to be only weakly correlated with the path loss fluctuations on the link between the MS to host BTS link. It is therefore possible that a MS, at anyone time and location, has a lower path loss to a different BTS than the one with which it is communicating.
In a conventional wireless network using the GSM standard, the base station controller (BSC) manages the radio link resources of the BTS. These resources are determined by the number of transceivers installed at the BTS and the number of radio channels anyone transceiver can handle. For example, in TDMA standards, a radio channel consists of a frequency and a time slot. In CDMA standards, a radio channel is represented by a frequency and one of a number of orthogonal spreading codes.
A BTS has two principal functions, that of controlling the radio links with all MSs within its cell, and relaying traffic between the BSC and the MSs. Relaying traffic includes receiving downlink traffic from the BSC and broadcasting it to MSs using broadcasters and that of receiving uplink traffic from the MSs using radio receivers called collectors and relaying it to the BSC.
In a mobile wireless network with a BSC, the BSC controls the assignment of the radio link resources (including Broadcasters and Collectors) in the BTSs as well as the operation of the network, and through the MSC, provides an interface with the Public Switched Telephone Network (PSTN). For generality, the BTS broadcasting and collecting functions can be considered as separate entities. In most existing networks, however, broadcasters (B) and collectors (C) are co-located.
In one example, three base transceiver stations (BTS) include three broadcasters and three collectors where broadcasters and collectors are typically but not necessarily co-located. The broadcasters and collectors have downlinks and uplinks to the BSC. These links are typically cabled links such as T1/E1 lines. The connection of these links between the broadcasters or collectors with the BSC may be arranged in various configurations such as a star pattern, a daisy-chain pattern or in any combination of these or other patterns.
When a connection is setup between a MS and the mobile network, a BSC selects the BTS that has the best radio access to the MS. This setup process includes a series of signal transmissions back and forth between the BSC, the BTSs, and the MSs using uplink and downlink radio control channels. The setup process results in the assignment of dedicated radio traffic and control channels for the uplinks and downlinks for communications between the MSs and the BTSs. Once these connections are set-up, user traffic, also called payload, can be transmitted between the MSs and the BSC. While the connection lasts, the BTS/BSC controls the operation of the radio traffic channels, including power control, frequency hopping, and timing advance. Also, the BTS/BSC continues to use the radio broadcast channels for operation, maintenance and signaling with all other MSs in its cell.
Users (MSs) communicate with collectors via control uplinks and traffic uplinks and with broadcasters via control downlinks and traffic downlinks. A particular broadcaster and collector is called the host broadcaster and the host collector for a particular MS. Together, they perform the function of the host BTS for the particular MS.
As MSs move within a cell and as the average path loss between an MS and its serving broadcaster and collector degrades, existing networks reassign the MS to another BTS (with a broadcaster and collector) that has a lower path loss. This process is called handover or handoff.
In wireless networks, dedicated radio links serve individual MSs and are at times operated at lower power levels. For instance, MSs close to a BTS do not require large transmit power levels and are operated at the minimum level meeting the link quality requirements. The reason for reducing power is to conserve radio band resources to enable reuse of radio resources in as many cells in the network as possible. MSs sharing uplink radio resources generate co-channel interference at their respective BTSs and BTSs sharing downlink radio resources generate co-channel interference at MSs.
Shadow fading imposes large fluctuations on the path loss between a particular MS moving in a cell and its serving BTS. At times when the path loss to a BTS is high, a high transmit power is used to maintain the quality of service. At such times, it is likely that the path loss between the particular MS and another BTS is lower because shadow fading effects between a MS and different BTSs are not highly correlated. Therefore, such other BTS can communicate traffic and/or control signals with the particular MS using lower uplink and downlink power levels. By switching the traffic and/or control channel over to such other BTS, the contribution of the particular radio link to the interference level in the network for other MS-BTS links that use the same radio resources is reduced. When such switching is implemented for many radio links in a network, a larger number of links can be operated in the network increasing network capacity without adding radio bandwidth.
The above-identified, cross-referenced application entitled SYSTEM FOR FAST MACRODIVERSITY SWITCHING IN MOBILE WIRELESS NETWORKS takes advantage of the de-correlation of shadow fading effects using fast macrodiversity switching (FMS) to select a BTS with the lowest instantaneous path loss for communicating uplink and downlink channels to a particular MS. In operation, host and assistant BTSs are employed. The host BTS remains in control of the particular MS via its broadcast channel until a handover is carried out. The dedicated channels with the particular MS are routed originally through the host BTS. When another BTS with a lower path loss becomes available, traffic and control channels are routed through such other BTS, which is designated as the assistant BTS for particular channels. As an MS moves through the cell, and as its path and shadow-fading losses change, the dedicated channels are switched among a number of BTSs in the network, including the host BTS. This fast macrodiversity switching continues unless the path loss between the particular MS and the host BTS becomes too high and a handover of the broadcast and dedicated channels is executed.
In the fast macrodiversity switching (FMS) process described, the radio resource used for a broadcast channel (frequency, time slot, code) for the host BTS is not changed while the dedicated channels are switched. The FMS process therefore differs from the handover process. Specifically, in the handover process, both the broadcast and dedicated channels are switched from radio resources assigned to the old BTS to radio resources assigned to the new BTS in accordance with a frequency reuse plan. By way of contrast in the FMS process, the broadcast channel is not switched while the dedicated channels are switched. The term “fast” is used to mean operations that are faster than the operations that are possible in a native protocol. For example, where the native protocol is GSM, the time scale of the FMS switching process is fast relative to switching for a handover. Fast macrodiversity switching operates, for example, at switching speeds of less than one second and in the range of 0.02 seconds to 0.25 seconds in a GSM embodiment. The FMS process can be done without modification to standard MS operation and also without changing the signaling to a MS.
In an FMS environment where timing advance is present, the combination of timing advance and fast macrodiversity switching causes portions of transmission to overlap and cause interference. When time advanced data is switched, the timing advance process is disturbed and will not operate, if at all, in the normal manner.
The European Telecommunications Standards Institute (ETSI) has defined a number of new GSM wireless data services. A GSM data call service at 14.4 kbits/sec per time slot is 50 percent higher than the standard 9.6 kbits/sec call service. A High Speed Circuit Switched Data (HSCSD) service aggregates several symmetric or asymmetric circuit channels at speeds of 28.8 kbits/sec when using two time slots (2+2) and 43.2 kbits/sec when using three time slots (3+1). Further extensions enable speeds of 56 kbits/sec, symmetrically (4+4) and asymmetrically (4+1). A General Packet Radio Service (GPRS) provides packet radio access to external Packet Data Networks such as the Internet. High spectrum efficiency is achieved by sharing time slots between different users with data rates of over 100 kbits/sec to a single user and a very low call set-up time. It offers direct IP connectivity, in a point-to-point or point-to-multipoint mode. An Enhanced Data Rate for GSM Evolution (EDGE) service modifies the modulation scheme used on the radio link from Gaussian Minimum Shift-Keying (GMSK)to Quadrature Amplitude Modulation (QAM). EDGE includes packet based technology E-GPRS (E-General Packet Radio Service) and circuit switched E-CSD (E-Circuit Switched Data) technology. Throughput is three times higher compared to standard GSM while using the same bandwidth. EDGE, has the potential of delivering data rates of over 300 kbits/sec to a single user.
The new services and technologies when employed with fast macrodiversity switching compound timing management problems. The problems include uplink burst overlap at the receiver of a BTS; downlink burst overlap at the transmitter of a BTS; interference due to burst overlap; shifting of slot receive boundaries at the BTS and at the MS and destruction of tail bits used for channel estimation. In (E)GPRS, the sharing of resources (time slots) between MS's, the need for scheduling and the need for resource allocation present difficulties that are further compound management requirements when fast macrodiversity is present.
Accordingly, there is a need for improved processing that permits fast macrodiversity switching in an environment of timing advance that helps achieve the objectives of improved performance and higher density of MSs.